The present invention relates to an optical information recording and reproducing apparatus in which a light spot is projected on a disc-shaped recording medium (rotary disc), whereby information such as a video signal is recorded or reproduced in the form of the change of any optical characteristic in the medium. More particularly, it relates to a technique according to which, when a track is moving reciprocally in the radial direction of a rotary disc due to the eccentricity of the disc, the same reciprocal motion is separately afforded to a light spot, thereby to facilitate a tracking pull-in operation for the spot.
In the optical information recording and reproducing apparatus as stated above, a rotary disc is used as a recording medium, and an information signal is recorded as a spiral recording locus or concentric recording loci on the rotary disc and is reproduced therefrom.
The concentric recording loci are suitable for the recording of information partitioned every fixed section, such as still picture information, whereas the spiral recording locus is suitable for the recording and reproduction of a continuous signal such as a video signal expressive of motion pictures or an audio signal. The recording locus will also be called `track` below.
In such information recording and reproducing apparatus, with the intentions of reducing the cost of the recording medium and making the size of the apparatus smaller, the density of recording in the recording medium will tend to become still higher in the future. To the accomplishment of the higher density, narrowing the track for a smaller track width has been more eargerly requested along with shortening a recording wavelength.
As one of problems attendant upon the progress of such narrowed tracks, when the recording medium with recorded information tracks after being detached from the apparatus has been set on the apparatus again, the eccentricity (the deviation between the center of recording and that of rotation) of the set recording medium attributed to a mechanical positional deviation or the plastic deformation of the recording medium attributed to a thermal or dynamical cause occurs to give rise to the deformation of the information track exceeding a track interval. For this reason, unless a tracking control following up the deformed shape of the information track is performed, the track position and the scanning position of reproduction by reproducing means (the scanning position of a light spot) involve a relative deviation in a direction traversing the tracks. Moreover, this deviation comes to fluctuate in magnitude.
Usually, among the deformations which arise in the information tracks, the deformation ascribable to the eccentricity accounts for the majority thereof. In this case, the fluctuation of the deviation magnitude mentioned above develops in synchronism with the rotation of the rotary disc being the recording medium, and the amplitude of the deviation magnitude and the phase of the fluctuating waveform relative to the rotational angle of the disc differ depending upon the set state of the disc.
The deviation magnitude of the information track due to the deformation arises with a value of several tens - several hundred .mu.m though it depends also upon the precision of the information reproducing apparatus or the disc itself, etc. It becomes one order or two orders greater than the track interval assumed to be about 2 .mu.m.
In the ordinary reproducing state of the disc, even when the magnitude of the deviation of the track in the radial direction of the disc with respect to the original reference position of the track is continually fluctuating due to the eccentricity of the rotary disc, the light spot is subjected to the tracking control so as to follow up the track, and hence, no problem is posed tentatively.
Meanwhile, it is sometimes wished to rapidly shift the light spot from a certain track position A on the rotary disc to another remote track position B. It is assumed by way of example that information to be obtained after reproducing recorded information at the track position A lie at the track position B spaced far therefrom. In this case, there is also a method in which the light spot is moved under the tracking control while reproducing information from the track position A to the position B. With this method, it takes a long time to arrive at the position B from the position A, and the desired information is obtained late.
If the light spot having performed the reproducing operation at the track position A till then is quickly shifted to the track position B with both the reproducing operation and the tracking control suspended and the reproducing operation is thereafter started at the position B, the desired information will be promptly obtainable.
However, when the reproducing operation with the light spot is to be started at the track position B after the shift, a problem takes place. More specifically, the track position B is not at rest but is continually reciprocating in the radial direction of the rotary disc on account of the eccentricity of the disc. Accordingly, the light spot having been shifted with a target at the reference position of the track position B (the original rest position) cannot immediately approach the track position B because of the continual movement thereof. After the shift, the search for the track position B is started. When the position B has been approached, the tracking control is turned on, whereupon the light spot follows up the track position.
As thus far described, when the rotary disc is not eccentric, the light spot is promptly accessible to the track position B, and it immediately turns on the tracking control. Thenceforth, it can follow up the track position. In contrast, in the case where the rotary disc is eccentric, there has heretofore been the problem that a period of time is required for the access to the track position B, so the desired information is obtained late.
One measure against the problem is a method wherein, as seen in the official gazette of Japanese Patent Application Laid-open No. 56-7247 by way of example, the waveform of a tracking signal in a tracking control corresponding to the distorted shape of an information track is stored beforehand, and reproducing means (a light spot) is subjected to the tracking control with the stored signal so as to depict a locus substantially agreeing with the distorted shape of the information track, thereby to correct the relative positional fluctuation between the reproducing means (the light spot) and the track and to shift a scanning position for reproduction (the position of the light spot) accurately and promptly.
It is premised for this method that the tracking control corresponding to the distorted shape of the information track proceeds reliably. However, when an eccentricity attributed to, e.g., a deviation involved at the setting of a disc is great, the tracking control following up the track distortion fails to proceed in some cases. This leads to the problem that the method has no effect in such cases.
Another problem is that an adverse effect is produced in a case where the waveform of the tracking signal is stored in the state in which the tracking is not normal and where the stored signal is used for the correction.